This invention broadly relates to an exposure technique used in a semiconductor process and, in particular, to a reflection type mask blank for EUV (Extreme Ultra Violet) exposure, a reflection type mask for EUV exposure, and a method of producing the same as well as a method of producing a semiconductor device using the same.
It is noted here that EUV light, which is described in the present specification, is a radiation having a wavelength within a soft X-ray region or a vacuum ultraviolet region, specifically, a wavelength within a range between about 0.2 and 100 nm.
In the semiconductor industry, an integrated circuit comprising fine patterns is formed on an Si substrate by the use of a pattern transfer technique. As the pattern transfer technique, use has typically been made of photolithography utilizing visible light or ultraviolet light. Following accelerated development of a semiconductor device having finer patterns, a shorter wavelength is required as an exposure wavelength to achieve higher resolution. On the other hand, limitation is imposed upon achievement of such shorter wavelength in existing optical exposure using the above-mentioned photolithography so that the limit of resolution is approaching.
In case of the photolithography, it is known that a pattern resolution limit is generally a half of the exposure wavelength. Even if an F2 laser beam having a wavelength of 157 nm is employed, it is predicted that the resolution limit is on the order of 70 nm. As an exposure technique capable of achieving the resolution beyond 70 nm, an EUV lithography (will be abbreviated to EUVL hereinafter) using EUV light is promising because the EUV light has a wavelength of 13 nm, much shorter than that of the F2 laser beam. The EUVL is similar to the photolithography in respect of the principle of image formation. For the EUV light, however, all substances exhibit high absorption and a refractive index is substantially equal to 1. Consequently, in the EUVL, a refraction optical system used in the photolithography can not be employed and, instead, a reflection optical system is exclusively used.
As a mask used in the EUVL, a transmission type mask with a membrane has been suggested recently. However, the transmission type mask is disadvantageous in that sufficient throughput can not be assured because the membrane exhibits high absorption for the EUV light and consequently an exposure time becomes long. Under the circumstances, a reflection type mask for exposure is generally used at present.
Referring to FIG. 1, description will briefly be made of first through third existing techniques for obtaining the above-mentioned reflection type mask for EUV exposure. Then, description will proceed to the necessity of an etching stopper used in a production process of the reflection type mask.
(First Existing Technique)
The production process for obtaining the reflection type mask for EUV exposure includes (1) a substrate preparation step of preparing a substrate, (2) a deposition step of depositing a multilayer film onto the substrate, (3) a deposition step of depositing an intermediate layer, (4) a deposition step of depositing an absorber layer, (5) an EB (Electron Beam) resist application step of applying an EB resist, (6) an EB resist writing step, (7) a dry-etching step, and (8) a removing step of removing the intermediate layer. Each of the above-mentioned steps will be explained hereinafter.
(1) Substrate Preparation Step:
Preferably, the substrate 11 has a low coefficient of thermal expansion and is superior in smoothness, flatness, and resistance to a cleaning method used for cleaning the EUV mask. As the substrate 11, a glass having a low coefficient of thermal expansion is generally used.
(2) Multilayer Film Deposition Step:
The multilayer film 12 contains Mo and Si in many cases.
For example, a single-period thickness is assumed to be 28 xc3x85 and 42 xc3x85 for Mo and Si, respectively. Then, by forming a laminate structure of at least 30 periods, it is possible to realize the multilayer film which reflects the EUV light having a peak wavelength of 13.4 nm. In case of the multilayer film containing Mo and Si, a Si film is deposited as a topmost layer.
(3) Intermediate Layer Deposition Step:
On the multilayer film 12 for reflecting the EUV light, an SiO2 film is deposited as the etching stopper constituting the intermediate layer. For example, the deposition may be carried out by RF magnetron sputtering using an SiO2 target.
(4) Absorber Layer Deposition Step:
The absorber layer 14 for absorbing the EUV light is deposited by sputtering. As a deposition material, Ta or Cr may be used. For example, the deposition may be carried out by DC magnetron sputtering. By this step, an EUV mask blank is obtained.
(5) EB Resist Application Step:
By forming a resist pattern on the absorber layer 14 of the EUV mask blank thus obtained, the EUV mask can be produced. The EB resist is applied on the EUV mask blank obtained in the step (4), and is baked at 200xc2x0 C.
(6) EB Resist Writing Step:
On the EUV mask blank with the EB resist applied thereon, the resist pattern is formed by the use of an EB writing machine.
(7) Dry-Etching Step:
With the above-mentioned resist pattern used as a mask, the EUV absorber layer 14 is dry-etched with chlorine to thereby form a pattern on the absorber layer.
(8) Intermediate Layer Removing Step:
The intermediate layer remaining on an EUV reflection surface, namely, the etching stopper 23 comprising the SiO2 film is removed with a dilute HF solution. Thus, the reflection type mask for EUV exposure is completed.
(Necessity of Etching Stopper and Problems Thereof)
The multilayer film 12 reflecting the EUV light must have high reflectivity after completion of the production of the mask. Therefore, it is required to prevent the multilayer film 12 reflecting the EUV light from being damaged during the production process. In particular, during the patterning step, the patterning must be performed without a damage, such as reduction in film thickness and roughening of the surface, given to the multilayer film 12.
In the patterning of the absorber layer 14 absorbing the EUV light, high dimensional accuracy can be obtained by dry-etching. However, it is impossible to perform the etching without damaging an underlayer of the absorber layer 14 absorbing the EUV light. In view of the above, it is necessary to deposit the etching stopper 23 as the intermediate layer between the multilayer film 12 and the EUV absorber layer 14.
As the etching stopper 23, use is generally made of an SiO2 film having a film thickness not smaller than several hundreds of angstroms. This film sufficiently serves as the stopper in the dry-etching with a Cl2 gas. However, If the SiO2 film remaining in a patternless area is not perfectly removed upon completion of the patterning step, the reflectivity of the multilayer film 12 reflecting the EUV light will be considerably lowered.
It is therefore necessary to perfectly remove the SiO2 film. However, if the dry-etching is carried out to remove the SiO2 film, the Si film as the uppermost layer of the multilayer film 12 reflecting the EUV light is inevitably etched. This also results in low reflectivity. For this reason, the SiO2 film must be removed by wet-etching with HF solution or the like. The wet-etching with the HF solution or the like is effective because no damage is given to the Si film as the underlayer of the SiO2 film. On the other hand, the wet etching with the HF solution has isotropic etchability so that the pattern is laterally eroded and may possibly be peeled off.
In addition, the SiO2 film having a film thickness not smaller than several hundreds angstroms has high surface roughness as well as high compression stress. Furthermore, abnormal discharge readily occurs during deposition of the SiO2 film by sputtering. It is therefore difficult to achieve a low fault rate required for the EUV mask.
(Second Existing Technique)
Japanese Unexamined Patent Publication (A) No. H08-213303 discloses a reflection type X-ray mask in which an intermediate layer containing Cr or Ti as a major component and having an etching ratio of 5 or more with respect to an absorber layer is formed on a multilayer film. According to this publication, the intermediate layer serves as an etching stopper as well as a protection layer for the multilayer reflection film when the pattern is formed on the absorber layer by etching. Alter the pattern is formed on the absorber layer, the intermediate layer positioned in a reflection region is removed.
(Third Existing Technique)
Japanese Unexamined Patent Publication (A) No. H07-333829 discloses a technique in which an intermediate layer is deposited between an absorber layer and a multilayer film by the use of a material (for example, Cr, Al, and Ni) having low absorption for exposure light, such as an X-ray and extreme ultra violet light, and having an etching rate slower than that of the absorber layer. Thus, it is possible to prevent decrease in reflectivity of the multilayer film without removing the intermediate film after the pattern is formed on the absorber layer by etching.
However, each of the intermediate films (SiO2 Cr, Al, Ni, and so on) described in conjunction with the first to the third existing techniques has a surface which is insufficient in smoothness and is roughened. Therefore, the absorber layer deposited on the intermediate layer having such a roughened surface also has a surface which is equally or more roughened as compared with the intermediate layer. As a result, the absorber pattern inevitably has a rough edge, giving an adverse affect to the transfer accuracy of the reflection type mask for EUV exposure.
Further, it is found out by the present inventors that, in the reflection type mask for EUV exposure as described in conjunction with the third existing technique among the first through the third existing techniques, i.e., in the reflection type mask having a structure in which the intermediate layer is left after formation of the pattern by etching, the roughened surface of the intermediate layer has a serious influence upon the transfer accuracy of the reflection type mask for EUV exposure.
Specifically, when a material (for example, Cr, Al, and Ni) having high surface roughness is employed as the intermediate layer left in the reflection region, the exposure light is scattered on the surface of the intermediate layer, resulting in decrease of the reflectivity.
Further, when the material of the intermediate layer, like Cr, Al, and Ni, is not resistant to a chemical agent used in a step of cleaning the reflection type mask for EUV exposure, degradation of the intermediate layer or separation of the pattern is caused to occur so that the reflection of the exposure light becomes non-uniform.
Moreover, when the material of the intermediate layer has a high film stress, the reflection surface of the reflection type mask for EUV exposure may be warped to thereby degrade the transfer accuracy of the pattern.
In the past, no consideration has been made about these problems, and no materials for solving these problems have not been found.
It is therefore an object of this invention to provide a reflection type mask blank for EUV exposure which is capable of forming a pattern with high accuracy and a reflection type mask for EUV exposure with high reflectivity as well as a method of producing the same.
It is another object of this invention to provide a method of producing a semiconductor device, which is capable of transferring a pattern on a semiconductor substrate by the use of the above-mentioned reflection type mask for EUV exposure having high reflectivity.
Other objects of this invention will become clear as the description proceeds.
According to a first aspect of this invention, there is provided a reflection type mask blank for EUV exposure comprising a substrate, a multilayer film formed on the substrate to reflect EUV light, an intermediate layer formed on the multilayer film, an absorber layer formed on the intermediate layer to absorb the EUV light, the intermediate layer being formed by a material containing Cr and at least one selected from the group consisting of N, O, and C.
According to a second aspect of this invention, the absorber layer in the mask blank of the first aspect of this invention is formed by a material containing Ta.
According to a third aspect of this invention, there is provided a reflection type mask blank for EUV exposure comprising a substrate, a multilayer film formed on the substrate to reflect EUV light, an intermediate layer formed on the multilayer film, an absorber layer provided with a pattern and formed on the intermediate layer to absorb the EUV light, the intermediate layer being formed by a material containing Cr and at least one element selected from the group consisting of N, O, and C.
According to a fourth aspect of this invention, the absorber layer in the mask blank of the third aspect of this invention is formed by a material containing Ta.
According to a fifth aspect of this invention, there is provided a method of producing a reflection type mask for EUV exposure by the use of the reflection type mask blank for EUV exposure of the first or the second aspect of this invention.
According to a sixth aspect of this invention, there is provided a method of producing a semiconductor device, the method comprising the step of transferring a pattern on a semiconductor substrate by the use of the reflection type mask for EUV exposure of the third or the fourth aspect of this invention.